Monday, June 01, 2015

Figure 1: Image of HD 115600 showing a bright debris
ring viewed nearly edge-on and located just beyond a Pluto-like distance
to its star. One or more unseen solar system-like planets are causing
the disk center to be offset from the star's position (cross). Figure
without labels is here. (Credit: Thayne Currie/NAOJ)

An international team led by Thayne Currie of the
Subaru Telescope and using the Gemini South telescope, has discovered a
young planetary system that shares remarkable similarities to our own
early solar system. Their images reveal a ring-like disk of debris
surrounding a Sun-like star, in a birth environment similar to the
Sun’s. The disk appears to be sculpted by at least one unseen solar
system-like planet, is roughly the same size as our solar system’s
Edgeworth-Kuiper Belt (commonly called the Kuiper Belt), and may contain
dust and icy particles. This work provides a valuable key to
understanding the early formation of the Sun and planets.

The discovery of the bright ring of orbiting the star
HD 115600 changes everything, said Currie, a Subaru Project Fellow
research astronomer. “It’s kind of like looking at outer solar system
when it was a toddler.”

Remarkably, the ring is almost exactly the same distance from its host star as the Kuiper Belt is from the Sun (Figure 1),
and it receives about the same amount of light. The star itself is
just slightly more massive than the Sun and is a member of a massive
grouping of 10- to 20-million-year-old stars called the
Scorpius-Centaurus OB association. Its birth cloud is very similar to
the nebula in which the Sun formed some 4.5 billion years ago.

There are strong indications that the ring around HD
115600 is being shaped by interactions with an unseen solar system-like
planet. The team measured the position of the ring with respect to the
star and found that the ring was significantly offset and has an
eccentric shape (meaning that it’s not very circular). This is likely
due to the gravitational effect of a massive planet. The calculated
eccentricity of the disk is among the largest known thus far, possibly
more than the ring around the planet-hosting star Fomalhaut (which has
at least one planet).

By using models that predict how planets of different
masses and orbital separations shape a debris disk, the team calculated
what kind of planet might be distorting HD 115600’s ring. They found
that eccentric versions of planets much like Jupiter, Saturn, Uranus, or
Neptune could explain the shape and other properties of the ring.

Other clues suggest that the ring may have a
composition similar to the Kuiper Belt. Its spectrum implies some types
of dust, as well as major Kuiper Belt constituents such as ice and
silicates. When compared with other debris disks, this one is much
more efficient at scattering starlight, which implies it has a
higher-reflecting, ice-like composition.

The discovery of the ring was made using the Gemini
Planet Imager (GPI), an instrument dedicated to detecting planets and
Kuiper Belt-like disks at never-before-seen scales. It is similar to the
Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument
currently being commissioned on the Subaru Telescope.

The results are very promising. “Even in just one of
our many 50-second exposures, we could see what previous instruments
failed to see in more than 50 minutes,” Currie said. “Given this
success with GPI, I’m very optimistic that Subaru’s own,
state-of-the-art planet-hunting instrument, SCExAO, will soon discover
many Kuiper belt-like disks and young planets and will put us well on
our way towards seeing another Earth.”

Comparing the Kuiper Belt to HD 115600’s Disk

Located just beyond Neptune’s orbit, the Kuiper
Belt contains numerous icy dwarf planets such as Pluto, Haumea, and
Makemake. It is also home to thousands of remnants from the earliest
stages of icy planet formation, and thus provides a key to understanding
the early solar system.

The study of cold, Kuiper belt-like debris rings
around nearby young Sun-like stars provides the best picture of what our
own early, outer solar system might have been like. However, the few
such rings that have been imaged so far haven’t always been similar to
ours. They usually surround stars much more massive than the Sun, or lie
at greater distances than the Kuiper Belt, or are located in sparse
star-forming regions unlike the massive and populous region in which the
Sun was born. Until now, studies of these disks lacked the
scattered-light spectra needed to explore them. Such studies can tell
give information about the structure of the ring, as well as its
motions.

The paper reporting these results is accepted for publication in The
Astrophysical Journal Letters with a title “Direct Imaging and
Spectroscopy of a Young Extrasolar Kuiper Belt in the Nearest OB
Association”
and can be found here.